[PATCH] inform user if any postinstall script failed to run

Here's a small patch for setup.exe which causes setup to indicate if a
postinstall script didn't run successfully.

This should help avoid the situation where the postinstall scripts fail to run
and the user has a broken installation, but they don't notice until they try
to run something which relies on postinstall scripts, e.g [1] and I'm sure
there are other examples.